Move PixelRefMap and its Iterator out from Picture class

cc/resources/picture_pile_impl.cc

 // Copyright 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include <algorithm>
 #include <limits>
 #include <set>
 
 #include "base/trace_event/trace_event.h"
 #include "cc/base/region.h"
@@ skipping 409 matching lines @@
         picture_pile_->picture_map_.find(tile_iterator_.index());
     if (it == picture_pile_->picture_map_.end())
       continue;
 
     const Picture* picture = it->second.GetPicture();
     if (!picture || (processed_pictures_.count(picture) != 0) ||
         !picture->WillPlayBackBitmaps())
       continue;
 
     processed_pictures_.insert(picture);
-    pixel_ref_iterator_ = Picture::PixelRefIterator(layer_rect_, picture);
+    pixel_ref_iterator_ = picture->GetPixelRefMapIterator(layer_rect_);
     if (pixel_ref_iterator_)
       break;
   }
 }
 
 void PicturePileImpl::DidBeginTracing() {
   std::set<const void*> processed_pictures;
   for (const auto& map_pair : picture_map_) {
     const Picture* picture = map_pair.second.GetPicture();
     if (picture && (processed_pictures.count(picture) == 0)) {
       picture->EmitTraceSnapshot();
       processed_pictures.insert(picture);
     }
   }
 }
 
 }  // namespace cc